The Three Methods of Charging
To electrify a neutral body we must add or remove one kind of charge — in solids, the loosely bound electrons. No new charge is ever created; charge is only transferred or redistributed, in keeping with the conservation of charge. The three standard routes differ in whether contact occurs and in the sign of the charge the body ends up with.
Friction (the historical route, NIOS §15.1) and conduction both involve the actual transfer of electrons and leave the body with the same sign of charge as its partner predicts. Induction stands apart: it relies only on the influence of a nearby charge and a connection to the earth.
| Method | Contact? | Mechanism | Sign acquired | Charging body |
|---|---|---|---|---|
| Friction | Yes (rubbing) | Electrons transfer between two dissimilar bodies rubbed together | Equal and opposite on the two bodies | Acquires opposite charge |
| Conduction | Yes (touch) | A charged body touches a conductor; charge shares across both | Same sign as the charging body | Loses some charge |
| Induction | No | Nearby charge redistributes the conductor's free electrons; earthing fixes the result | Opposite sign to the inducing body | Charge unchanged |
Friction works for insulators (a plastic comb, a glass rod) because the transferred charge stays put. Conduction and induction require a conductor — a material in which electrons are free to move, as a metal, the human body, or the earth. The remainder of this page develops induction, the only method in which the body is charged without contact.
Charging a Conductor by Induction
Consider an isolated metal sphere mounted on an insulating stand, and a positively charged rod. The goal is to leave the sphere with a net charge without ever touching it with the rod. The procedure is a strict ordered sequence — change the order and it fails.
- Bring the rod near. Hold the positive rod close to the sphere without touching it. The rod's field acts on the sphere's free electrons.
- Charges redistribute. Free electrons are attracted toward the rod, accumulating on the near face (negative), leaving the far face electron-deficient (positive). The sphere is still neutral overall — only separated.
- Earth the far side. Connect the far face to the ground (touch it, or run a wire to earth) while the rod stays in place. The repelled positive end is now free to be neutralised: electrons flow up from the earth to cancel it, so the like (positive) charge effectively drains away. The unlike (negative) charge stays bound near the rod.
- Remove earth, then rod. First break the earth connection — this traps the negative charge on the sphere. Only then remove the rod. The bound electrons now spread over the whole surface, leaving the sphere with a net negative charge.
Throughout, the rod never touched the sphere and parted with none of its own charge. The net charge on the sphere came entirely from the earth. A positive rod yields a negatively charged conductor; reversing to a negative rod would yield a positively charged conductor.
Induction gives the OPPOSITE charge — and needs no contact
Students reflexively assume the body ends up with the same sign as the charging agent (true only for conduction). In induction the conductor acquires the opposite sign, because the unlike charge is the one that stays trapped after earthing.
Positive rod → negatively charged conductor. Negative rod → positively charged conductor.
The order of removal is not optional
If the rod is removed before the earth connection is broken, the trapped electrons simply flow back to the earth and the sphere returns to neutral. Earthing must be removed first, while the rod still holds the unlike charge in place. Skip earthing entirely and the sphere is only polarised — removing the rod lets the separated charges recombine.
Induction works only in materials where charge can move. Revisit Conductors and Insulators to see why a metal redistributes charge while glass cannot.
Induction in a Two-Sphere Setup
A neater demonstration uses two identical metal spheres on insulating stands, touching each other. Bringing a positive rod near one end induces a negative charge on the near sphere and a positive charge on the far sphere. While the rod is held in place, the spheres are separated; then the rod is removed. The result is two oppositely charged spheres — one negative, one positive — of equal magnitude, conjured without any contact with the rod.
This version requires no earth: separating the spheres while the rod holds the charges apart plays the same role that earthing did for a single conductor — it locks in the separation before the inducing field is withdrawn. By conservation of charge the two induced charges are equal in magnitude and opposite in sign, summing to zero, exactly as the spheres began.
Why the Induced Charge is Opposite
The sign is dictated by the direction free electrons move under the rod's influence. A positive rod attracts electrons toward itself, so the near face turns negative — unlike the rod. Earthing then bleeds off the far (like) charge, leaving the near, unlike charge behind. The rule is therefore not a convention but a direct consequence of attraction between unlike charges.
This same asymmetry explains why a charged body attracts a neutral conductor. The induced unlike charge sits on the near face, closer to the rod; the like charge sits on the far face, farther away. Since the Coulomb force falls with distance, the attraction on the near charge exceeds the repulsion on the far charge, and the net force is attractive — even though the conductor carries no net charge.
The inducing rod loses no charge
A favourite distractor: "after charging a sphere by induction, what is the charge on the rod?" Because there is no contact, the rod's charge is exactly what it was at the start. The sphere's net charge is supplied by the earth, not by the rod.
Polarisation of an Insulator
An insulator has no free electrons, so it cannot acquire a net charge by induction the way a conductor does — there is no charge to drain to the earth. Yet a charged rod still attracts an uncharged insulator (a comb lifting paper). The mechanism is polarisation.
In an external field the bound charges within each molecule shift slightly — positive nuclei one way, electron clouds the other — turning each molecule into a tiny induced dipole. The displacements are minute and no charge actually separates from the material, but across the whole slab the result is a layer of negative charge on the face toward a positive rod and positive on the far face. This induced surface charge produces the same net attraction.
The contrast is essential for NEET: a conductor produces free charge separation that can be made permanent through earthing, whereas a dielectric produces only bound-charge dipoles that vanish the instant the external field is removed.
Polarisation is not "charging" the insulator
An insulator in a field has induced dipoles, not a net charge. Remove the field and the slab is neutral and unpolarised again. Do not equate a polarised dielectric with a body charged by induction — only conductors retain a net induced charge.
Charging by Induction — at a glance
- Three methods: friction (transfer, opposite signs), conduction (contact, same sign), induction (no contact, opposite sign).
- Induction sequence: bring rod near → electrons redistribute → earth the far side → remove earth → remove rod.
- The conductor ends with charge opposite to the rod; the rod's charge is unchanged.
- Earthing is essential; remove it before the rod, or the charge escapes back to earth.
- Two-sphere version: separate the touching spheres while the rod is held, giving equal and opposite charges.
- Insulators only polarise — bound charges shift into induced dipoles, no free charge separation, no permanent net charge.